Single-photon absorptiometry has been widely accepted for biomedical research and clinical assessment of bone mineral in vivo. Dual-photon absorptiometry is being developed in our laboratory for measurements of fluid shifts and of spinal and whole-body bone mineral. The current absorptiometric techniques for bone mineral and fluid content have a clinical precision of 2-3%. This precision is limited by (a) the time required to mechanically scan over a limb (30 sec) or whole body (70 min), (b) the effects of local bone irregularities on linear scans of long bones (distal radius and ulna), and (c) the representativeness of a linear scan at one site on a bone. We propose to overcome problems of reproducibility and representativeness, and to shorten measurement times, by developing a method using an area detector fo absorptiometry. The precision of bone mineral or fluid shift measurement will be increased to 0.5%; this would permit assessment of disease or of therapy over shorter periods and will increase representativeness. Also, area absorptiometry of fluid shifts will allow (a) assessment of venous status and (b) monitoring of fluid shifts experienced by trauma patients. The reasonable cost, plus the availability, physical characteristics and technological documentation leads us to propose area absorptiometry using a multiwire proportional counter (MWPC). The first year of the project will involve construction of MWPC, associated electronics and the testing of the equipment. Clinical studies with the MWPC will begin in the second year. Area absorptiometry of bone mineral and fluid shifts will be done on limbs and the spine. In the third year of the project whole-body absorptiometry using the MWPC will be applied to studies of normals and patients.